Joining between composite and metallic or thermoplastic components is currently approached in a number of ways, each with its own limitations.
The use of fasteners is commonplace but tends to result in de-lamination around fastener holes, as well as the associated difficulties of drilling holes in composites such as Carbon Fiber and Aramids such as Kevlar.
The bearing strength of laminated composites tends to be low, as does the inter-laminar shear strength. This results in a requirement for significant reinforcement around fastener holes, leading to a large weight increase, which is particularly undesirable in aerospace applications.
Fastened joints tend to be particularly weak in the pull-through direction (that is, the direction of axial load through the fastener) and as such are not well suited to aerospace applications such as fastening ribs to covers, where air and fuel pressure loads tend to result in significant axial component of load through the fastener.
Adhesive bonds are an increasingly common means of joining metallic components to composite laminates, however these perform poorly in peel, tension and cleavage, and tend to fail with little or no warning. Their weakness in peel and in tension makes bonded joints similarly limited in their application within conventional aerospace structures. Any mitigation for the poor performance in peel or tension tends to result in large bond surface areas, with the associated weight penalties that go with this.
Existing research into the use of surface features to improve the strength of metallic/composite joints is limited.
WO 2004/028731 A1 describes a method by which surface features are generated by using a ‘power-beam’ such as an electron beam, in order to ‘flick-up’ surface material on a metallic component to sculpt protruding features that are intended to increase bond surface area and improve bond strength when incorporated into the matrix of a co-cured laminate.
This process has certain limitations.
Firstly, the process displaces surface material to create the protruding features. This could lead to surface damage of the component, and is likely to generate crack initiators that will adversely affect the fatigue life of the parent part.
Secondly, the process does not provide scope for optimising the profile and shape of the protrusions, which could produce significant improvements in the performance of the joint—particularly in tension and peel.
Thirdly, the process is an additional step in the production of a component and as such has an adverse time and cost impact.
Fourthly, the method can only form protruding features with a relatively low aspect ratio.